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Using a Scan Tool for Transmission Diagnosis

A scan tool can be an effective transmission diagnostic tool. But to understand what’s happening inside the trans, you must be able to properly interpret the raw sensor data the scanner provides.

Communicating with a vehicle’s on-board diagnostic system is essential when trying to diagnose transmission problems. And the way to do that is with a scan tool, a device that gives the eyes needed to determine the appropriate diagnostic approach. Although the tool may provide both information and bidirectional control, the data retrieved still needs to be processed and interpreted.

To make full use of a scan tool, one must understand all the abilities and options it has to offer. No one should expect a generic scan tool to be equivalent to a factory scan tool. The former offers coverage over a broader range of manufacturers, while the latter is designed with a focus on the needs of a single manufacturer. The trade-off is obvious: The manufacturer’s specialized scan tool can do more but covers only that manufacturer’s vehicles, while the generic scan tool may have fewer functions but covers more vehicles. Since most shops work on a slew of different makes, a generic scan tool becomes a necessity.

One of the challenges for a scan tool manufacturer is deciding what information to put in and what to leave out. And when it comes to automatic transmission data, it becomes an even greater challenge on several levels. For example, the 4L60-E transmission used in many General Motors vehicles began in 1993. During the first two years, this transmission had a pressure switch manifold to inform the computer of the manual valve’s position, two shift solenoids, a pressure control solenoid, a 3-2 pulse width modulated (PWM) downshift solenoid, a transmission converter clutch (TCC) On/Off solenoid and a vehicle speed sensor. In 1995, GM added a PWM TCC solenoid. In this way, the TCC On/Off solenoid turns the converter clutch on and off while the PWM TCC solenoid controls the rate at which it’s applied and released. The very next year (1996), the 3-2 PWM solenoid was changed to function as an On/Off solenoid. As these changes were occurring, computer strategies were changing as well, which in turn affected the data parameters being offered to scan tools. And it didn’t stop there.

In 1997, the converter clutch control strategy was changed to what’s known as the Electronically Controlled Capacity Clutch (EC3). It was first introduced in W-body 3.4L passenger cars and then in all GM models in 1998. The purpose of the EC3 converter clutch operation is to provide a controlled slip as early as 2nd gear to a rare full apply of the clutch during specific highway cruise conditions. This continuous low-rpm converter clutch slip still allowed for improved fuel economy while further enhancing driveability by reducing driveline torsional disturbances.

After this strategy was in place, the operation of the 4L60-E remained the same from 1998 to 2005. But in 2006, GM decided to add an input shaft speed (ISS) sensor to the transmission for improved gear monitoring and pressure control, creating a new data parameter for the scan tool. Finally, in 2009, GM eliminated the 3-2 On/Off downshift solenoid and replaced the pressure switch manifold with an internal mode switch. This eliminated the 3-2 downshift solenoid data parameter while changing the pressure switch manifold data to a signal now provided by an internal mode switch.

Such changes not only challenge the scan tool companies in providing data parameters appropriate to various vehicles, they also challenge the technician who has to diagnose those vehicles. If the tech is not aware of these variations, the data provided by the scan tool can be confusing. He may wonder why it is that with one vehicle a 100% converter clutch apply occurs while with another it rarely goes to full apply. Or why a 3-2 downshift solenoid shows 0% in 1st to 90% in 2nd, 3rd and 4th and about 6% on a 3-2 downshift, yet with another vehicle it always reads ON and goes to OFF on a 3-2 downshift.

I’ve used GM’s 4L60-E as an example to stress the importance of knowing changes made throughout the years with any transmission being diagnosed. Without having knowledge of changes made to the control system, interpretation of scan tool data can be challenging. Having this knowledge gives you the edge needed for successful diagnoses, but it also makes you aware of the data parameters missing and how to interpret the ones that are provided, especially in terms of how a signal is generated. For example, there are many types of transmission range sensors that signal to the computer the location of the shift lever. One way this is accomplished is with several wires through which the computer supplies voltage to the sensor, which grounds them in various combinations.

The open and closed signals the computer uses to analyze the shift lever position are also provided to the scan tool. If one or two of these signals remain open or closed, one way to optimize the use of a scan tool is to first turn the ignition off, then unplug the transmission range sensor and turn the ignition on again. Use a voltmeter to verify that voltage is being supplied to the sensor from the computer and that the scan tool is reporting all circuits open. Then, by grounding each of these circuits at the transmission range sensor’s harness connector, the scan tool should show closed for the circuit being grounded. If it does, yet when you plug the sensor back in the problem immediately returns, you’ve verified the need for a new sensor.

Another helpful aspect of a scan tool is the use of rpm data, particularly if the transmission is equipped with an input speed sensor. I mention this because rpm data is a parameter that often makes it onto the list of PIDs in generic scan tools, and this can be extremely helpful, especially since many later model transmissions are equipped with input and output rpm sensors. With engine, input and output rpm data available, gear ratio and converter clutch slip can be calculated. In some cases, if a transmission is slipping, the specific clutch assembly causing the slip can be determined before the tech has to pull the unit.

Calculating gear ratio is best done from a recorded movie, preferably one that captures as many shifts as possible from a 1st gear start (see Fig. 4 on page 20). By reviewing each frame, you can divide the output rpm reading into the input rpm reading to obtain the current gear ratio for that frame. In most cases, when the converter clutch is fully applied, engine rpm should match input rpm. Subtracting input rpm from engine rpm provides TCC slip data. Many four-speed transmissions have a 1:1 3rd gear ratio. By canceling Overdrive, the converter clutch will apply in 3rd. The result should be that all three rpm sensors read the same.

This is exactly what the computer looks at to determine gear ratio and converter clutch slip percentage. But let’s take this a step further using a 2001 Ford Windstar 3.8L VIN 4 with a front-wheel-drive AX4N transmission (a similar but newer version of the AX4S transmission). This is another example of the importance of knowing how a signal is generated so you can properly interpret the data provided in the scan tool.

This transmission has all three rpm sensors available to be used for diagnostics, but the input rpm signal is generated with a slight twist. Being a front-wheel-drive transmission, the torque from the engine through the torque converter drives a turbine shaft and drive sprocket (Fig. 5 on page 22). The drive sprocket/turbine shaft then supplies this torque input to the driven sprocket via a chain. The ISS is located in such a fashion that it’s excited by a four-tooth sensor wheel mounted on the driven sprocket (Figs. 6 and 7). The twist here is that these two sprockets (Fig. 8) vary in the number of teeth between them and between the various vehicles they’re in. This means the input speed sensor rpm data is different from the actual speed of the drive sprocket, and the overall sprocket ratio is matched to the vehicle’s computer system. But knowing that the actual rotation of the drive sprocket is different from the rotation of the driven sprocket makes sense of the data observed from a scan tool.

Getting back to the 2001 Windstar example, this vehicle has a 38/39 drive/ driven sprocket set. Dividing the driv­en sprocket tooth count into the drive sprocket tooth count, the overall sprocket ratio is calculated to be .9743589:1. Multiplying this number by engine rpm gives the speed of the driven sprocket.

Using the screen capture shown in Fig. 9 on the previous page, multiplying .9743589 by engine speed (1619.4) yields 1577.8768. This is really the rpm of the driven sprocket, but the scan tool will display this as the rpm speed of the turbine shaft (drive sprocket). The scan tool is reporting a turbine speed of 1583.00, which is approximately a 6-rpm difference from our calculations. This is considered perfect, as the data captured in the scan tool is not real time. There’s a bit of a delay between the actual rpm input to the computer and the time this information reaches the scan tool over the data lines.

If the tech looking at this data is unaware as to how this system works, he may think the scan tool is offering erroneous data. Notice that this screen capture (Fig. 9) is indicating that the converter clutch is fully applied (100%) and the TCC slip ratio is 1:1. Usually, when the converter clutch is fully applied, the engine rpm and turbine rpm are identical. It would appear that there’s a 36-rpm slip (161921583), but in this case there is not. The turbine rpm reading is really the driven sprocket rpm, but by knowing the overall ratio of the drive and driven sprocket set (as does the computer), the actual rotation of the turbine shaft is really 1618.

One error that occurs from time to time is when an incorrect overall sprocket ratio is used. This causes the computer to interpret the data as a converter clutch apply problem. Often the transmission will upshift to 4th gear without incident. As the converter clutch is commanded on, the computer recognizes an error in the rpm change and attempts to apply the clutch four consecutive times before aborting the command. Usually harsh shifts follow, with a converter clutch performance code set. On some occasions, depending on the type of misapplication, a wrong sprocket ratio can be quickly identified by looking at these rpm readings. A 0-rpm slip is reached at 40% to 50% duty cycle of the solenoid, followed by a sudden increase in rpm slip as the solenoid duty cycle continues to increase.

Another quick calculation that can be made using the same screen capture is to divide the output rpm into the turbine rpm (1619.4 4 2289.5 = .070). This will provide the gear ratio of the transmission, which in this example indicates a good 4th gear as an AX4N transmission has a .69:1 Overdrive ratio.

Dodge/Chrysler has an interesting test made available through the use of rpm sensors and bidirectional control of the solenoids to diagnose a slipping 41TE (A604) transmission. It’s referred to as the Clutch Test. When selecting this test, after reading safety warnings and instructions provided by the scan tool, you arrive at a display that offers four different pairs of clutch assemblies to apply (Fig. 10). Once a pair of clutches are selected, you’re prompted to brake-torque the vehicle no higher than 20% throttle for no longer than 5 seconds. During this time, the turbine rpm sensor should remain at 0 (Fig. 11). If the scan tool provides any rpm reading other than 0, the transmission is slipping.

To determine which clutch is slipping, a different pairing of clutch assemblies is selected and, by process of elimination, a specific clutch assembly is identified as being faulty before the unit needs to be removed. For example, three of the four available pairing of clutch assemblies are the Underdrive and Low/Reverse, the Underdrive and 2/4 and the Underdrive and Overdrive. If a test for the first selection resulted in a 0 rpm, yet in the second selection resulted in a 95 rpm reading, the 2/4 clutch assembly would be the clutch assembly at fault.

This type of test helps determine whether or not there’s a genuine fault inside the transmission. But this Clutch Test is available only with specific Dodge and Chrysler transmissions. For other manufacturers and transmissions, a good work-around is to use a scan tool in conjunction with a transmission shift box. Simply set the tool to view input rpm data and use the shift box to manually select a gear. View the input rpm data during a 20% brake torque in each gear to determine, by process of elimination, which component is slipping. Although this type of testing requires the use of a transmission shift box, if this option is available it can quickly indicate whether the transmission is slipping or the wrong gear ratio transmission was installed.

Unlike the 41TE transmission, where all the planetary gear sets have the same ratios, a gear ratio error in other transmissions could mean the wrong transmission was used. GM, for example, has front-wheel-drive transmissions with several different overall gear ratios that if incorrectly used will cause the computer to produce gear ratio codes. Performing a Clutch Test with the use of a scan tool and a shift box would immediately show that none of the internal components is slipping, thus pointing to the possibility of a wrong ratio transmission being installed.

These are just a few tips (especially using rpm data) on how to optimize the use of a scan tool when diagnosing transmission-related problems. With the cost of scan tools today, it makes sense to squeeze all you can out of one. More importantly, though, using a scan tool can help you solve a transmission-related problem as quickly as possible.